Team building help please!

I don't think Emiya(Assassin) fits in this team if you are trying to make use out of Lancelot. My suggestion would be to switch Emiya(Assassin) with Hans since he will contribute a lot more to the team.

It really depends on what you're trying to do. If you want np spam, then hans/martha, emiya, mash is a solid start line. For a more buster/card damage oriented team, try lubu, lancelot, mash(for defensive support). David is also decent as a support(and some decent damage) if you need an archer. Arash is mostly used for nuking/farming.